refactor(web): split CheckoutView into checkout-view module
- types.ts: CheckoutViewProps, CheckoutFormState, INITIAL_CHECKOUT_FORM
- useCheckoutView: form state, cart/tax, handlePurchase, onComplete
- CheckoutViewHeader, CheckoutViewBillingCard, CheckoutViewPaymentCard,
CheckoutViewOrderSummary, CheckoutViewSkeleton
- CheckoutView orchestrator; re-export from CheckoutView.tsx
- Stories: Loading (Skeleton), Default, Processing, Success
- Decorator min-h-screen -> min-h-layout-page
Co-authored-by: Cursor <cursoragent@cursor.com>
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